1. Field of the Invention
The present invention relates to a projection exposure method and an apparatus therefor and, more particularly, to a method of the exposure by projection for transcribing or transferring a pattern of a mask to a photosensitizable substrate at a high degree of sensitivity by an apparatus for the method for the exposure by projection to be employed in a photolithographic process for manufacturing semiconductor elements, liquid crystal displays, image pickup elements (CCD and so on), thin film magnetic heads, and so on. This projection exposure method is adapted particularly to a method for correcting an error in a nonlinear magnification.
2. Description of the Related Art
For a photolithographic process for manufacturing semiconductor elements, liquid crystal display elements, image pickup elements, thin film magnetic heads and so on, there has hitherto been employed mainly a projection exposure apparatus of a stepper type so adapted as to collectively expose a pattern of a reticle, a photomask or the like to each shot area on a photosensitizable substrate such as a photoresist-coated wafer or glass plate via a projection optical system. On the other hand, there has recently been employed a projection exposure apparatus of a slit-scanning type or of a step-and-scanning type or the like, so adapted as to expose a pattern of a reticle one by one onto each of shot areas on a wafer by scanning the reticle and the wafer relative to a projection optical system, in order to meet requirements for exposure of a pattern having a larger area while sustaining high imaging characteristics.
These projection exposure apparatuses must be able to provide a high degree of accuracy in performing alignment. For example, as different circuit patterns are superimposed in plural layers on a wafer for semiconductor elements and so on, it is required that a reticle with a circuit pattern drawn thereon should be aligned accurately with a pattern of each shot area on the wafer in order to subject the circuit pattern of the reticle to exposure to the pattern on the wafer. In this case, an error in magnification (distortion) of a projection optical system should be taken into consideration as a significant factor that may adversely affect accuracy in alignment. Further, as the tendency to miniaturize a larger pattern to be employed for super LSIs and so on has been increased year after year, the demand for improvements in accuracy in alignment of a pattern on a wafer has been strongly increased, accordingly. Therefore, the necessity of sustaining a magnification of projection of the projection optical system within a given scope has also been strengthened remarkably.
The magnification of projection of a projection optical system varies to a slight extent in the vicinity of a predetermined value, for example, with a minute variation in temperature of an apparatus, a minute variation in pressure and temperature of the ambient atmosphere in a clean room with a projection exposure apparatus installed therein, a history of irradiation of energy of light exposed to the projection optical system and so on. Therefore, the magnification of projection of the projection optical system may readily be distorted when such factors for affecting the magnification of projection of the projection optical system are deviated to a slight extent from the acceptable predetermined level. Further, distortion of the magnification of projection by the projection optical system may be caused to occur due to thermal expansion of a reticle or for other reasons. In order to sustain a variation in a magnification of projection of the projection optical system within a predetermined acceptable scope, projection exposure apparatuses developed recently are provided with a projection magnification correction mechanism for sustaining a variation in the magnification of projection in the vicinity of the predetermined acceptable value by minutely adjusting the magnification of projection of the projection optical system. As such projection magnification correction mechanisms, there have been proposed a mechanism of varying a distance between a reticle and a projection optical system, a mechanism of varying a predetermined distance between lenses in a projection optical system, a mechanism of adjusting a predetermined pressure in an air chamber disposed in a projection optical system, and so on.
Where the magnification of projection of the projection optical system may be corrected by such a projection magnification correction mechanism as described hereinabove, there has hitherto been employed a type, for example, in which an error in magnification at a height of 70% of the maximum image height is corrected by using a predetermined reference image height having 70% of the maximum image height. However, there are types of distortion in which an error in magnification changes in a nonlinear shape in accordance with an image height. Among such types of nonlinear distortions, there is a so-called C-shaped distortion in which an error in magnification occurs in a C-letter shape with respect to a change in image height and such C-shaped distortion can prevent error correction. Therefore, recently, a projection exposure apparatus may further be provided with a correction mechanism to correct C-shaped distortion.
It is to be noted herein that in the prior art techniques, the method of magnification error correction, for example, to zero with a one predetermined reference image height is effective for correction of a linear distortion, but it cannot effectively correct linear distortion. Further, disposition of a correction mechanism for correcting C-shaped distortion makes the structure of a projection optical system complicated and increases costs.